Catalysts are employed in the exhaust systems of automotive vehicles to convert carbon monoxide, hydrocarbons, and nitrogen oxides (NOx) produced during engine operation into non polluting gases. When the engine is operated in a stoichiometric or slightly rich air/fuel ratio, i.e., between about 14.7 and 14.4, catalysts containing precious metals like palladium and rhodium are able to efficiently convert all three gases simultaneously. Hence, such catalysts are often called "three-way" catalysts.
It is desirable, however, to operate gasoline engines under "lean-burn" conditions where the A/F ratio is greater than 14.7, generally between 19 and 27, to realize a benefit in fuel economy. Fuel efficient diesel engines also operate under A/F ratios greater than 19, generally 19-40. Such three-way catalysts are able to convert carbon monoxide and hydrocarbons but are not efficient in the reduction of NOx during lean-burn (excess oxygen) operation. Efforts have thus been made in developing SRC in recent years. Such catalysts act to reduce the NOx through the use of hydrocarbons over a catalyst, the hydrocarbons being in turn oxidized.
Often these catalysts are based on zeolite materials containing a precious metal like platinum which can have major drawbacks. Among the most important are a narrow temperature range of operation and loss of activity (and sometimes physical integrity) under the hydrothermal conditions of automotive exhaust gases. For example, a zeolite material carrying platinum is generally only active at a relatively low temperature, i.e., less than 250.degree. C. At higher temperatures the competitive oxidation of the reductant hydrocarbon molecules by oxygen is so fast that the removal of NOx drops off precipitously with rising temperature so as to make such catalyst inadequate for treating somewhat hotter exhaust streams. Conversely, when the active sites are transition metal ions exchanged into the cationic sites of the zeolite, the onset of SCR activity begins at temperatures greater than 400.degree. C., which renders the catalyst inactive for catalysis during a large portion of the necessary temperature range of the desired catalytic operation. And the stability of an automotive catalyst in dry gases is insufficient to assure the proper long-term function in the presence of steam, as is the case in automotive exhaust. Exposed to such conditions the zeolite becomes "dealuminated" whereby the cationic sites where the divalent exchanged ions are anchored are destroyed.
We have found that these problems associated with conventional zeolites can be overcome with the catalyst of the present invention. In particular, the invention catalyst is a selective reduction catalyst, stabilized against dealumination and resistant to poisons such as sulfur dioxide and water vapor and active in the temperature range pertinent particularly to use in diesel powered vehicles and vehicles powered by other lean-burn power plants.